ObjectiveFatty acid-binding proteins (FABPs) are a family of 14-15-kDa proteins, and some FABPs have been to be used as biomarkers of tissue injury by leak from cells. However, recent studies have shown that FABPs can be secreted from cells into circulation. Here we examined determinants and roles of circulating FABPs in a general population.MethodsFrom the database of the Tanno-Sobetsu Study, a study with a population-based cohort design, data in 2011 for 296 subjects on no medication were retrieved, and FABP1∼5 in their serum samples were assayed.ResultsLevel of FABP4, but not the other isoforms, showed a gender difference, being higher in females than in males. Levels of all FABPs were negatively correlated with estimated glomerular filtration rate (eGFR), but a distinct pattern of correlation with other clinical parameters was observed for each FABP isoform; significant correlates were alanine aminotransferase (ALT), blood pressure (BP), and brain natriuretic peptide (BNP) for FABP1, none besides eGFR for FABP2, age, BP, and BNP for FABP3, age, waist circumference (WC), BP, BNP, lipid variables, high-sensitivity C-reactive protein (hsCRP), and HOMA-R for FABP4, and age, WC, BP, ALT, BNP, and HOMA-R for FABP5. FABP4 is the most strongly related to metabolic markers among FABPs. In a multivariate regression analysis, FABP4 level was an independent predictor of HOMA-R after adjustment of age, gender, WC, BP, HDL cholesterol, and hsCRP.ConclusionsEach FABP isoform level showed a distinct pattern of correlation with clinical parameters, although levels of all FABPs were negatively determined by renal function. Circulating FABP4 appears to be a useful biomarker for detecting pre-clinical stage of metabolic syndrome, especially insulin resistance, in the general population.

BackgroundRepresentatives of the phylum Chloroflexi, though reportedly highly abundant in the extensive deep water habitats of both marine (SAR202 up to 30% of total prokaryotes) and freshwater (CL500-11 up to 26% of total prokaryotes), remain uncultivated and uncharacterized. There are few metagenomic studies on marine Chloroflexi representatives, while the pelagic freshwater Chloroflexi community is largely unknown except for a single metagenome-assembled genome of CL500-11.ResultsHere, we provide the first extensive examination of the community composition of this cosmopolitan phylum in a range of pelagic habitats (176 datasets) and highlight the impact of salinity and depth on their phylogenomic composition. Reconstructed genomes (53 in total) provide a perspective on the phylogeny, metabolism, and distribution of three novel classes and two family-level taxa within the phylum Chloroflexi. We unraveled a remarkable genomic diversity of pelagic freshwater Chloroflexi representatives that thrive not only in the hypolimnion as previously suspected, but also in the epilimnion. Our results suggest that the lake hypolimnion provides a globally stable habitat reflected in lower species diversity among hypolimnion-specific CL500-11 and TK10 clusters in distantly related lakes compared to a higher species diversity of the epilimnion-specific SL56 cluster. Cell volume analyses show that the CL500-11 are among the largest prokaryotic cells in the water column of deep lakes and with a biomass to abundance ratio of two they significantly contribute to the deep lake carbon flow. Metabolic insights indicate participation of JG30-KF-CM66 representatives in the global cobalamin production via cobinamide to cobalamin salvage pathway.ConclusionsExtending phylogenomic comparisons to brackish and marine habitats suggests salinity as the major influencer of the community composition of the deep-dwelling Chloroflexi in marine (SAR202) and freshwater (CL500-11) habitats as both counterparts thrive in intermediate brackish salinity; however, freshwater habitats harbor the most phylogenetically diverse community of pelagic Chloroflexi representatives that reside both in epi- and hypolimnion.Electronic supplementary materialThe online version of this article (10.1186/s40168-018-0563-8) contains supplementary material, which is available to authorized users.

The oxygenated hypolimnion accounts for a volumetrically significant part of the global freshwater systems. Previous studies have proposed the presence of hypolimnion-specific bacterioplankton lineages that are distinct from those inhabiting the epilimnion. To date, however, no consensus exists regarding their ubiquity and abundance, which is necessary to evaluate their ecological importance. The present study investigated the bacterioplankton community in the oxygenated hypolimnia of 10 deep freshwater lakes. Despite the broad geochemical characteristics of the lakes, 16S rRNA gene sequencing demonstrated that the communities in the oxygenated hypolimnia were distinct from those in the epilimnia and identified several predominant lineages inhabiting multiple lakes. Catalyzed reporter deposition fluorescence in situ hybridization revealed that abundant hypolimnion-specific lineages, CL500-11 (Chloroflexi), CL500-3, CL500-37, CL500-15 (Planctomycetes) and Marine Group I (Thaumarchaeota), together accounted for 1.5–32.9% of all bacterioplankton in the hypolimnion of the lakes. Furthermore, an analysis of single-nucleotide variation in the partial 16S rRNA gene sequence (oligotyping) suggested the presence of different sub-populations between lakes and water layers among the lineages occurring in the entire water layer (for example, acI-B1 and acI-A7). Collectively, these results provide the first comprehensive overview of the bacterioplankton community in the oxygenated hypolimnion of deep freshwater lakes.

Uncultured bacteria affiliated with the CL500‐11 cluster (phylum Chloroflexi) were first reported from the oxygenated hypolimnion of Crater Lake (USA) as a predominant bacterioplankton, although this dominance has not been reported in other environments. In this study, we showed that CL500‐11 is also dominant in the oxygenated hypolimnion of Lake Biwa (Japan) and followed its spatiotemporal succession using fluorescent in situ hybridization. CL500‐11 cells were almost absent [< 1% of 4′,6‐diamidino‐2‐phenylindole (DAPI)‐stained cells] at the beginning of the stratification period, dominated (> 10% of DAPI‐stained cells; maximum = 16.5%) in the hypolimnion during the stratification period, and decreased to below the detection limit with the collapse of the thermocline. This pattern was observed over two annual cycles. A longitudinal assessment also showed that CL500‐11 was the dominant bacterium in the hypolimnion over the whole lake, but was generally undetectable in the stratified epilimnion. These data suggest that CL500‐11 is acclimated to the oxygenated hypolimnion and is a potentially important component of the pelagic biogeochemical cycling of the lake. A comparative analysis of 16S rRNA gene sequences revealed that almost all CL500‐11 sequences previously deposited in the database were detected from hypolimnion or holomictic water in deep oxic freshwater lakes, suggesting that the bacteria may form one of the common lineages residing in an aerobic hypolimnetic niche.

In freshwater microbial ecology, extensive studies are attempting to characterize the vast majority of uncultivated bacterioplankton taxa. However, these studies mainly focus on the epilimnion and little is known regarding the bacterioplankton inhabiting the hypolimnion of deep holomictic lakes, despite its biogeochemical importance. In this study, we investigated the bacterioplankton community composition in a deep freshwater lake with a fully oxygenated hypolimnion (Lake Biwa, Japan) using high-throughput 16S rRNA gene amplicon sequencing. Sampling at a pelagic site over 15 months throughout the water column revealed that the community composition in the hypolimnion was significantly different from that in the epilimnion. The bacterial community in the hypolimnion was composed of groups dominating in the whole water layer (e.g., bacI-A1 and acI-B1) and groups that were hypolimnion habitat specialists. Among the hypolimnion specialists, members of Chloroflexi and Planctomycetes were highly represented (e.g., CL500-11, CL500-15 and CL500-37), followed by members of Acidobacteria, Chlorobi and nitrifiers (e.g., Ca. Nitrosoarchaeum, Nitrosospira and Nitrospira). This study identified the number of previously understudied taxa dominating the deep aerobic freshwater habitat, suggesting that the biogeochemical cycling there is driven by the microbial community that are different from that in the epilimnion.